1. Field of the Invention
The present invention relates to a pressure sensor that includes a sensor diaphragm configured to output a signal corresponding to a difference in received pressure between one surface thereof and the other surface thereof.
2. Description of the Related Art
A pressure sensor that includes a sensor diaphragm configured to output a signal corresponding to a difference in received pressure between one surface thereof and the other surface thereof has been used as an industrial pressure sensor (see, e.g., Japanese Unexamined Patent Application Publication No. 10-300612).
This pressure sensor is configured to guide the pressure of a measured fluid received on a pressure receiving diaphragm to one surface of the sensor diaphragm using a pressure transmitting medium (enclosed liquid), such as silicone oil, detect a strain in the sensor diaphragm caused by the difference in pressure from other surface as, for example, a change in resistance value in a strain resistance gauge, convert the change in resistance value to an electric signal, and extract the electric signal.
FIG. 6 illustrates a main part of a pressure sensor of the related art. In FIG. 6, reference numeral 1 denotes a metal body, reference numeral 2 denotes a pressure receiving diaphragm, reference numeral 3 denotes an enclosing chamber formed inside the body 1, reference numeral 4 denotes a sensor chip placed in the enclosing chamber 3, and reference numeral 5 (5-1, 5-2) denotes electrode pins.
In a pressure sensor 100, the body 1 is formed by a base body 1-1 and a cover body 1-2, and the enclosing chamber 3 is formed by a pressure receiving chamber 3-1, a pressure guiding passage 3-2, and a sensor chamber 3-3. The pressure receiving diaphragm 2 is secured by welding an outer edge thereof to the upper surface of the base body 1-1. The pressure receiving chamber 3-1 is formed on the back surface of the pressure receiving diaphragm 2. The pressure receiving chamber 3-1 communicates with the sensor chamber 3-3 through the pressure guiding passage 3-2. An enclosed liquid 6 is enclosed in the enclosing chamber 3 formed by the pressure receiving chamber 3-1, the pressure guiding passage 3-2, and the sensor chamber 3-3.
The sensor chip 4 is formed by a sensor diaphragm 4-1, and a first retaining member 4-2 and a second retaining member 4-3 joined to each other with the sensor diaphragm 4-1 interposed therebetween. The sensor diaphragm 4-1 is a sheet-like member made of, for example, silicon or glass. The sensor diaphragm 4-1 is provided with a strain resistance gauge formed on a surface thereof. In FIG. 6, the surface of the sensor diaphragm 4-1 having the strain resistance gauge formed thereon is indicated by diagonal lines.
The retaining members 4-2 and 4-3 are also made of silicon, glass, or the like. The first retaining member 4-2 has a recessed portion 4-2a and a pressure introducing hole (pressure guiding hole) 4-2b communicating with the recessed portion 4-2a, and the second retaining member 4-3 has a recessed portion 4-3a and a pressure introducing hole (pressure guiding hole) 4-3b communicating with the recessed portion 4-3a. The recessed portion 4-2a of the first retaining member 4-2 has a flat bottom surface, whereas the recessed portion 4-3a of the second retaining member 4-3 has a curved (aspheric) bottom surface along displacement of the sensor diaphragm 4-1.
The first retaining member 4-2 is jointed to one surface (first surface) 4-1a of the sensor diaphragm 4-1, with a surrounding region 4-2c of the recessed portion 4-2a facing the first surface 4-1a of the sensor diaphragm 4-1. The second retaining member 4-3 is joined to the other surface (second surface) 4-1b of the sensor diaphragm 4-1, with a surrounding region 4-3c of the recessed portion 4-3a facing the second surface 4-1b of the sensor diaphragm 4-1.
In the pressure sensor 100, the sensor chip 4 is placed in the sensor chamber 3-3. A bottom surface 4a of the sensor chip 4 (or a lower surface of the second retaining member 4-3) having an epoxy adhesive applied thereto is joined to a bottom surface 3a of the sensor chamber 3-3 (or an inner wall surface of the cover body 1-2). That is, the bottom surface 4a of the sensor chip 4 and the bottom surface 3a of the sensor chamber 3-3 are joined to each other, with a layer (adhesive layer) 7 of epoxy adhesive material interposed therebetween. The cover body 1-2 has a pressure guiding passage 1-2a at a position corresponding to the pressure guiding hole 4-3b in the second retaining member 4-3 of the sensor chip 4. The pressure guiding passage 1-2a is a passage for introducing an atmospheric pressure into the pressure guiding hole 4-3b. 
In the pressure sensor 100, the areas of respective surfaces of the first retaining member 4-2 and the second retaining member 4-3 facing each other, with the sensor diaphragm 4-1 interposed therebetween, are different. In this example, the sensor chip 4 has a stepped structure in which the area of the surface of the first retaining member (upper retaining member) 4-2 facing the second retaining member (lower retaining member) 4-3 is smaller than the area of the surface of the second retaining member 4-3 facing the first retaining member 4-2. In the stepped structure of the sensor chip 4, wires 8 (8-1, 8-2) are configured to extend from the surface of the sensor diaphragm 4-1 having the strain resistance gauge thereon, at an outer edge of one of the first retaining member 4-2 and the second retaining member 4-3, the one (which is the second retaining member 4-3 in this example) extending outward from the other retaining member. The wires 8 (8-1, 8-2) extending from the sensor diaphragm 4-1 are connected to the respective electrode pins 5 (5-1, 5-2).
The electrode pins 5 each pass through the cover body 1-2, with one end portion thereof positioned inside the sensor chamber 3-3 and the other end portion thereof positioned outside the sensor chamber 3-3. The cover body 1-2 has insertion holes 1-2b, which allow the respective electrode pins 5 to pass through the cover body 1-2. For electrical insulation between the cover body 1-2 and each of the electrode pins 5, as well as for prevention of leakage of the enclosed liquid 6, the insertion holes 1-2b in the cover body 1-2 are hermetically sealed with a sealing material 9.
In the pressure sensor 100, a pressure P1 from a measured fluid (fluid, gas) is received by the pressure receiving diaphragm 2. The pressure P1 of the measured fluid received by the pressure receiving diaphragm 2 is transmitted to the enclosed liquid 6 in the enclosing chamber 3, passes through the pressure receiving chamber 3-1, the pressure guiding passage 3-2, and the sensor chamber 3-3 to enter the pressure guiding hole 4-2b in the first retaining member 4-2, and is guided to the first surface 4-1a of the sensor diaphragm 4-1. The second surface 4-1b of the sensor diaphragm 4-1 is open to the atmosphere through the pressure guiding hole 4-3b in the second retaining member 4-3.
This causes a strain in the sensor diaphragm 4-1. The strain in the sensor diaphragm 4-1 is detected as a change in resistance value in the strain resistance gauge. The change in resistance value is converted to an electric signal (i.e., a signal corresponding to a difference in received pressure between the first surface 4-1a and the second surface 4-1b), transmitted through the wires 8 (8-1, 8-2), and extracted from the electrode pins 5 (5-1, 5-2).
If the sensor diaphragm 4-1 is displaced by an excessive pressure applied to the first surface 4-1a of the sensor diaphragm 4-1, the entire displaced surface is received by the curved surface of the recessed portion 4-3a of the second retaining member 4-3. This prevents excessive displacement caused by the excessive pressure applied to the sensor diaphragm 4-1, reduces stress concentration on the outer region of the sensor diaphragm 4-1 to prevent the sensor diaphragm 4-1 from being broken by the application of the excessive pressure, and enhances the resistance of the sensor diaphragm 4-1 to pressure.